Besides, i believe black widows have a red marking on their upperside.
Mother Nature added the warning sign on the underside, which is uppermost as the spider usually hangs upside-down in its web.
The female Black Widow is shiny black, usually with a reddish hourglass shape on the underside of her spherical abdomen. Her body is about 1.5 inches long. Adult males are harmless, about half the female's size, with smaller bodies, longer legs and usually have yellow and red bands and spots over the back as do the immature stages.
- from http://www.desertusa.com/july97/du_bwindow.html
When I was a teenager hand hoeing weeds on our eastern Washington farm, I picked up the carcass from an old tire that was lying on the ground, intending to sit on its upper tread while I took a break. Just as I started raising it, I noticed just a few inches from my fingers in the tires interior, this large shiny black arachnid.
Needless to say I dropped the tire forthwith. After I investigated and saw that red hourglass, I proceeded to disembody the critter with my hoe.